Add swapBuffer call to SkGLContextHelper.

src/gpu/gl/win/SkNativeGLContext_win.cpp

Index: src/gpu/gl/win/SkNativeGLContext_win.cpp
diff --git a/src/gpu/gl/win/SkNativeGLContext_win.cpp b/src/gpu/gl/win/SkNativeGLContext_win.cpp
index 4c736994fe20441e682c8b014c691faec8945469..d8777274bcc6925a1dfffa73446888890961f129 100644
--- a/src/gpu/gl/win/SkNativeGLContext_win.cpp
+++ b/src/gpu/gl/win/SkNativeGLContext_win.cpp
@@ -112,3 +112,9 @@ void SkNativeGLContext::makeCurrent() const {
         SkDebugf("Could not create rendering context.\n");
     }
 }
+
+void SkNativeGLContext::swapBuffers() const {
+    if (!SwapBuffers(fDeviceContext)) {
+        SkDebugf("Could not complete SwapBuffers.\n");
+    }
+}